Method for driving liquid crystal display apparatus

ABSTRACT

In one embodiment of the present invention, when a still image is displayed, applied voltages respectively corresponding to a total of n (n being an integer of not less than 4) types of gradation  0  to (n−1) are outputted to pixels. When a moving image is displayed, an applied voltage corresponding to a predetermined gradation m (1≦m≦(n−2)) is applied to the pixels instead of applied voltages respectively corresponding to gradations of less than the predetermined gradation m. Overdrive driving is performed with respect to a total of n types of gradation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.13/488,801 filed Jun. 5, 2012, which is a Divisional of U.S. applicationSer. No. 11/887,220 filed Sep. 27, 2007, which claims priority under 35U.S.C. §§120, 121 to PCT Application No. PCT/JP2006/302076, filed Feb.7, 2006, which claims priority to Japanese Application No. 2005-104862,filed Mar. 31, 2005, the entire contents of each of which are hereinincorporated by reference.

TECHNICAL FIELD

The present invention relates to methods for driving liquid crystaldisplay apparatuses. Particularly, the present invention relates to amethod for driving a liquid crystal display apparatus, which methodmakes it possible to achieve an improvement in response speed at which amoving image is displayed.

BACKGROUND ART

Conventionally, a liquid crystal display apparatus has had a problem oflow response speed. That is, a change in display gradation of the liquidcrystal display apparatus is such that: a change in orientation state ofliquid crystal molecules is made by making a change in voltage appliedto a liquid crystal layer, so that the transmittance of a display pixelis changed. Moreover, the low response speed of the liquid crystaldisplay apparatus is attributed to the fact that it takes a long time tocomplete the change caused in orientation state of the liquid crystalmolecules in response to the change in voltage applied to the liquidcrystal layer.

In recent years, liquid crystal display apparatuses such as liquidcrystal televisions, portable televisions, and portable game machineshave had increased opportunities to display high-definition movingimages with liquid crystals, and therefore have been increasinglyrequired to respond at high speeds. On the other hand, high-qualitypicture technologies often cause a decrease in response speedsimultaneously (e.g., AVS and mobile AVS).

As disclosed in Japanese Unexamined Patent Application Publication No.78129/2004 (Tokukai 2004-78129; published on Mar. 11, 2004), a knownexample of a method for attempting to improve response speed is a methodfor emphasizing a transitional gradation by performing overdrivedriving. That is, as shown in FIG. 12, the overdrive driving is suchthat when the initial luminance A of the initial gradation 0 is changedto the target luminance C of the target gradation 64, a voltagecorresponding to the excessive luminance B, which is higher than thetarget luminance C, is initially applied to the liquid crystals only fora short time. This causes a high voltage to be applied to the liquidcrystals, thereby making it possible to reduce the response time ittakes to attain the target luminance C.

However, as shown in FIG. 9, such a method causes deterioration in imagequality. Examples of such deterioration in image quality include aso-called angular response (two-step response) which, before the targetluminance C is attained, emerges as a sharp corner indicating theexcessive luminance B, which is higher than the target luminance C. Thepresence of such a corner indicating a luminance higher than the targetluminance C causes an image to instantaneously look whitish. Since thisis very conspicuously identified, it is necessary that the driving beperformed so that no such corner emerges.

However, a change in overdrive amount only causes a change in size ofthe left angular portion, and does not result in an improvement in theright sloping portion. Therefore, there is no improvement in display.Further, as described above, an excessive overdrive amount causes astrikingly white display to be produced at the angular portion, therebycausing deterioration in display quality.

Furthermore, even when the overdrive driving is performed, a sufficientspeed may not be able to be obtained in a low-gradation region due tothe aforementioned low response speed.

That is, the aforementioned low response speed of a liquid crystaldisplay apparatus is not seen uniformly all over the gradation-levelregions, but is such that the response speed becomes extremely low inpart of the gradation regions. For example, the response speed of avertically-aligned and normally black liquid crystal display apparatus(mobile ASV) is extremely low at a rising edge from a low gradation(black display) to an intermediate gradation. Further, the responsespeed of a normally white liquid crystal display apparatus (mobile ASV)is extremely low in a transition from a high gradation (white display)to an intermediate gradation. These low response speeds cause displayproblems such as residual images.

In view of this, for example, Japanese Unexamined Patent ApplicationPublication No. 131721/2002 (Tokukai 2002-131721; published on May 9,2002) discloses a method for improving response speed by carrying out adisplay without using a gradation level at which the response speedbecomes low. Specifically, the liquid crystal driving method of PatentDocument 2 tries not to use a gradation level, ranging from a highgradation (white display) to an intermediate gradation, at which theresponse speed of a normally white system becomes low. Note that avoltage so applied to liquid crystals to be used for driving a liquidcrystal display apparatus is usually represented by agradation-luminance curve shown in FIG. 13.

However, according to the conventional method of Tokukai 2002-131721 fordriving a liquid crystal display apparatus, the initial voltage isincreased by a predetermined voltage when a gradation level at which theresponse speed becomes low is not used. Therefore, when a still image isdisplayed, it is impossible to use a normal luminance characteristicrepresented by the gradation-luminance curve of FIG. 13.

The present invention has been made in view of the foregoing problems,and it is an object of the present invention to provide a method fordriving a liquid crystal display apparatus, which method makes itpossible to prevent deterioration in display quality of both a stillimage and a moving image and to achieve an improvement in response speedat which a moving image is displayed.

DISCLOSURE OF INVENTION

In order to solve the foregoing problems, a method of the presentinvention for driving a liquid crystal display apparatus includes thesteps of: when a still image is displayed, outputting applied voltagesto pixels, the applied voltages respectively corresponding to a total ofn (n being an integer of not less than 4) types of gradation 0 to (n−1);and when a moving image is displayed, (i) outputting an applied voltageto the pixels instead of applied voltages respectively corresponding togradations of less than a predetermined gradation m (1≦m≦(n−2)) and (ii)performing overdrive driving with respect to the total of n types ofgradations, the applied voltage corresponding to the predeterminedgradation m.

According to the foregoing invention, normal gradations can be displayedwhen a still image is displayed. On the other hand, when a moving imageis displayed, an applied voltage corresponding to a predeterminedgradation m (1≦m≦(n−2)) is applied to pixels instead of applied voltagesrespectively corresponding to gradations of less than the predeterminedgradation. Therefore, the applied voltages respectively corresponding tothe gradations of less than the predetermined gradation m are not used.This makes it possible to achieve an improvement in response speed.

Furthermore, the present invention performs overdrive driving withrespect to the total of n (n being an integer of not less than 4) typesof gradation. Therefore, the applied voltages respectively correspondingto the gradations of less than the predetermined gradation m are notused when overdrive driving is performed. This makes it possible toprevent a so-called angular response.

This makes it possible to provide a method for driving a liquid crystaldisplay apparatus, which method makes it possible to preventdeterioration in display quality of both a still image and a movingimage and to achieve an improvement in response speed at which a movingimage is displayed.

Further, in order to solve the foregoing problems, a method of thepresent invention for driving a liquid crystal display apparatusincludes the steps of: when a still image is displayed, outputtingapplied voltages to pixels, the applied voltages respectivelycorresponding to a total of n (n being an integer of not less than 4)types of gradation 0 to (n−1); when a moving image is displayed, withoutusing applied voltages respectively corresponding to gradations of lessthan a predetermined gradation m (1≦m≦(n−2)), overlapping (n−m) types ofgradation partially so that n gradations are obtained and then sortingthe n gradations into a range of (i) an applied voltage corresponding tothe predetermined gradation m to (ii) an applied voltage correspondingto the gradation (n−1); and when an applied voltage corresponding to agradation k (k being an integer of 0 to (n−1)) obtained by the sortingis applied to the pixels, performing overdrive driving with respect tothe total of n (n being an integer) types of gradation.

According to the foregoing invention, the applied voltages respectivelycorresponding to the gradations of less than the predeterminedgradations m (1≦m≦(n−2)) are not used when a moving image is displayed.As a result, a low-gradation display is not carried out in a normallyblack system. This causes a range of luminances that can be displayed tobe narrower than when normal display driving is performed, therebycausing deterioration in display quality.

In view of this, the present invention overlaps (n−m) types of gradationpartially so that n gradations are obtained and then sorts the ngradations into a range of (i) an applied voltage corresponding to thepredetermined gradation m to (ii) an applied voltage corresponding tothe gradation (n−1). Therefore, even when the applied voltagesrespectively corresponding to the gradations of less than thepredetermined gradation m are not used, the n gradations can beexpressed. This makes it possible to prevent deterioration in displayquality. Further, since the overdrive driving is performed, the responsespeed is increased.

Further, in order to solve the foregoing problems, a method of thepresent invention for driving a liquid crystal display apparatusincludes the steps of: when a still image is displayed, outputtingapplied voltages to pixels, the applied voltages respectivelycorresponding to a total of n (n being an integer of not less than 4)types of gradation 0 to (n−1); when a moving image is displayed, withoutusing applied voltages respectively corresponding to gradations of lessthan a predetermined gradation m (1≦m≦(n−2)), redividing the total of ntypes of gradation within a range of the predetermined gradation m to agradation (n−1); and when an applied voltage corresponding to agradation p (p being an integer of 0 to (n−1)) obtained by theredivision is applied to the pixels, performing overdrive driving withrespect to the total of n types of gradation.

According to the foregoing invention, the applied voltages respectivelycorresponding to the gradations of less than the predeterminedgradations m (1≦m≦(n−2)) are not used when a moving image is displayed.As a result, a low-gradation display is not carried out in a normallyblack system. This causes a range of luminances that can be displayed tobe narrower than when normal display driving is performed, therebycausing deterioration in display quality.

In view of this, the present invention redivides the total of n types ofgradation within a range of the predetermined gradation m to a gradation(n−1). Therefore, even when the applied voltages respectivelycorresponding to the gradations of less than the predetermined gradationm are not used, the n gradations can be expressed. This makes itpossible to prevent deterioration in display quality. Further, since theoverdrive driving is performed, the response speed is increased.

Further, in order to solve the foregoing problems, a method of thepresent invention for driving a liquid crystal display apparatusincludes the steps of: when a still image is displayed, outputtingstill-image applied voltages to pixels, the still-image applied voltagesrespectively corresponding to a total of n (n being an integer of notless than 4) types of gradation 0 to (n−1); when a moving image isdisplayed, without using applied voltages respectively corresponding togradations of less than a predetermined gradation m (1≦m≦(n−2)), (i)outputting, to the pixels, applied voltages, corresponding to a range ofthe gradation 0 to a gradation (n−1), which are obtained by adding, toeach of the still-image applied voltages, an applied voltagecorresponding to the predetermined gradation m, and (ii) performingoverdrive driving with respect to the total of n types of gradation.

According to the foregoing invention, the applied voltages respectivelycorresponding to the gradations of less than the predeterminedgradations m (1≦m≦(n−2)) are not used when a moving image is displayed.As a result, a low-gradation display is not carried out in a normallyblack system. This causes a range of luminances that can be displayed tobe narrower than when normal display driving is performed, therebycausing deterioration in display quality.

In view of this, the present invention outputs, to the pixels, appliedvoltages corresponding to a range of the gradation 0 to a gradation(n−1), the applied voltages being obtained by adding, to each of thestill-image applied voltages, an applied voltage corresponding to thepredetermined gradation m. Therefore, even when the applied voltagesrespectively corresponding to the gradations of less than thepredetermined gradation m are not used, n gradations can be expressed.This makes it possible to prevent deterioration in display quality.Further, since the overdrive driving is performed, the response speed isincreased.

Further, in order to solve the foregoing problems, a method of thepresent invention for driving a liquid crystal display apparatusincludes the steps of: when a still image is displayed, outputtingapplied voltages to pixels, the applied voltages respectivelycorresponding to a total of n (n being an integer of not less than 4)types of gradation 0 to (n−1); and when a moving image is displayed, (i)outputting an applied voltage to the pixels instead of applied voltagesrespectively corresponding to gradations of not less than apredetermined gradation q (1≦q≦(n−1)) and (ii) performing overdrivedriving with respect to the total of n types of gradation, the appliedvoltage corresponding to the predetermined gradation q.

Further, in order to solve the foregoing problems, a method of thepresent invention for driving a liquid crystal display apparatusincludes the steps of: when a still image is displayed, outputtingapplied voltages to pixels, the applied voltages respectivelycorresponding to a total of n (n being an integer of not less than 4)types of gradation 0 to (n−1); when a moving image is displayed, withoutusing applied voltages respectively corresponding to gradations of notless than a predetermined gradation q (1≦q≦(n−1)), overlapping (n−q)types of gradation partially so that n gradations are obtained andsorting the n gradations into a range of an applied voltagecorresponding to the predetermined gradation q−1 to an applied voltagecorresponding to the gradation 0; and when an applied voltagecorresponding to a gradation k (k being an integer of 0 to (n−1))obtained by the sorting is applied to the pixels, performing overdrivedriving with respect to the total of n types of gradation.

Further, in order to solve the foregoing problems, a method of thepresent invention for driving a liquid crystal display apparatusincludes the steps of: when a still image is displayed, outputtingapplied voltages to pixels, the applied voltages respectivelycorresponding to a total of n (n being an integer of not less than 4)types of gradation 0 to (n−1); when a moving image is displayed, withoutusing applied voltages respectively corresponding to gradations of notless than a predetermined gradation q (1≦q≦(n−1)), redividing the totalof n types of gradation within a range of the predetermined gradationq−1 to a gradation 0; and when an applied voltage corresponding to agradation p (p being an integer of 0 to (n−1)) obtained by theredivision is applied to the pixels, performing overdrive driving withrespect to the total of n types of gradation.

Further, in order to solve the foregoing problems, a method of thepresent invention for driving a liquid crystal display apparatusincludes the steps of: when a still image is displayed, outputtingstill-image applied voltages to pixels, the still-image applied voltagesrespectively corresponding to a total of n (n being an integer of notless than 4) types of gradation 0 to (n−1); when a moving image isdisplayed, without using applied voltages respectively corresponding togradations of not less than a predetermined gradation q (1≦q≦(n−1)), (i)outputting, to the pixels, applied voltages, corresponding to a range ofthe gradation 0 to a gradation (n−1), which are obtained by adding, toeach of the still-image applied voltages, an applied voltagecorresponding to the predetermined gradation q, and (ii) performingoverdrive driving with respect to the total of n types of gradation.

Additional objects, features, and strengths of the present inventionwill be made clear by the description below. Further, the advantages ofthe present invention will be evident from the following explanation inreference to the drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an embodiment of a method of the present invention fordriving a liquid crystal display apparatus, and is a characteristicdiagram showing a gradation-luminance relationship formed when alow-gradation region is eliminated at the time of displaying a movingimage.

FIG. 2 is a block diagram showing an overall arrangement of the liquidcrystal display apparatus.

FIG. 3 is a waveform chart showing a response waveform obtained when alow-gradation region is eliminated at the time of displaying a movingimage in the liquid crystal display apparatus and of performingoverdrive driving.

FIG. 4( a) is a diagram showing a relationship between time andgradation data that are to be written in pixels when overdrive drivingis performed such that the gradation 0 (black) in the previous frame ischanged to a gradation 128 (intermediate gradation) for the currentframe.

FIG. 4( b) is a waveform chart showing a liquid-crystal responsewaveform obtained from FIG. 5( a).

FIG. 5 is a diagram showing a look-up table, in which overdrive drivingoutput data are stored in correspondence with the gradation values ofprevious-frame image data and the gradation values of current-frameimage data, of the liquid crystal display apparatus.

FIG. 6 is a characteristic diagram showing a gradation-luminancerelationship formed when n gradations are sorted into a range ofvoltages for gradations (n−m) or when the same range of applied voltagesis redivided in accordance with n gradations at the time of displaying amoving image in the liquid crystal display apparatus.

FIG. 7 is a diagram showing converted gradations and liquid crystalapplied voltages each used when first to third methods are employed inthe liquid crystal display apparatus.

FIG. 8 is a characteristic diagram showing, in contrast to a normalgradation-luminance relationship, a gradation-luminance relationshipformed when n gradations are sorted into a range of voltages forgradations (n−m) or when the same range of applied voltages is redividedin accordance with n gradations at the time of displaying a moving imagein the liquid crystal display apparatus.

FIG. 9 is a characteristic diagram showing a gradation-luminancerelationship formed when a backlight adjustment is made at the time ofdisplaying a moving image in the liquid crystal display apparatus.

FIG. 10 shows another embodiment of a method of the present inventionfor driving a liquid crystal display apparatus, and is a characteristicdiagram showing a gradation-luminance relationship formed when anapplied voltage is shifted.

FIG. 11 is a characteristic diagram showing a gradation-luminancerelationship formed when a backlight adjustment is made at the time ofdisplaying a moving image according to the method for driving a liquidcrystal display apparatus.

FIG. 12 shows a conventional method for driving a liquid crystal displayapparatus, and is a waveform chart showing overdrive driving.

FIG. 13 is a characteristic diagram showing a normal gradation-luminancerelationship of the liquid crystal display apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

[Embodiment 1]

An embodiment of the present invention will be described below withreference to FIGS. 1 to 9.

For example, as shown in FIG. 2, an active matrix liquid crystal displayapparatus 10 of the present embodiment includes: a display section 1, agate driving section 2, a source driving section 3, a common electrodedriving section 4, a control section in which a calculating section 5 isprovided, a frame memory 7, a look-up table 8, and a backlight drivingsection 9.

Although not fully shown in the figure, the display section 1 includes escanning signal lines that are parallel to one another, f data signallines that are parallel to one another, and pixels arrayed in a matrixmanner. Each of the pixels is formed in a region enclosed by twoadjacent scanning signal lines and two adjacent data signal lines.

The gate driving section 2 sequentially generates, in accordance with agate clock signal and a gate start pulse each outputted from the controlsection 6, scanning signals that are to be supplied to scanning signallines connected to pixels arrayed in each line.

The source driving section 3 samples an image data signal DAT inaccordance with a source clock signal and a source start pulse eachoutputted from the control section 6, and outputs the obtained imagedata to data signal lines connected to pixels arrayed in each line.

The control section 6 is a circuit that generates, in accordance with asynchronization signal inputted thereto, the image data signal DAT, anda signal MS for discriminating between an moving image and a stillimage, various control signals for controlling operation of the gatedriving section 2 and source driving section 3, and then outputs thecontrol signals. As described above, examples of the control signalsthat are outputted from the control section 6 include the clock signal,the start pulse, and the image data signal DAT.

The calculating section 5 of the control section 6 converts the imagedata signal DAT when a moving image is displayed. The data conversion inthe calculating section 5 is performed, for example, in accordance withdata stored in the look-up table 8. Note that the calculating section 5can be integrated with a driver such as the source driving section 3 orthe gate driving section 2. Further, in cases where an external controlIC is provided, the calculation section 5 can also be part of thatcontrol IC. Furthermore, the calculating section 5 can also beincorporated as a monolithic circuit into the display section 1.Further, according to the foregoing example, the calculating section 5is provided in the control section 6. However, the present invention isnot limited to this. It is also possible to perform a gradation processor the after-mentioned black process by disposing only the calculatingsection 5 in front of the control section 6.

On this occasion, the control section 6 receives a signal MS fordiscriminating between a moving image and a still image, therebydetermining whether or not a moving image is displayed. In case of astill image, the control section 6 becomes able to carry out a displaywithout making a gradation transition, and therefore becomes able tocarry out a display without impairing γ characteristic, luminance, andcontrast at all.

Such a signal MS for discriminating between a moving image and a stillimage can be realized, for example, by preparing a single terminal foran input signal in such a way that a high input signal indicates amoving image and a low input signal indicates a still image. That is,for example, it is possible that: the control section 6 receives, from auser set, a 1-bit signal that represents a moving image or a stillimage, thereby discriminating between a moving image and a still image.

Note that the discrimination between a moving image and a still image isnot necessarily limited to this. For example, a command that representsa moving image or a still image may be received. Furthermore, it ispossible to employ a discriminating method including the steps of:storing, in the frame memory 7, data corresponding to the previous framethat came immediately before the current frame; comparing the data withdata corresponding to the current frame; and setting a moving-image modewhen there is a difference between the data corresponding to theprevious frame and the data corresponding to the current frame. Examplesof the difference between the data corresponding to the previous frameand the data corresponding to the current frame include a difference ofnot less than a predetermined gradation and a difference of not lessthan a certain number of pixels.

Meanwhile, each of the pixels of the display section 1 includes aswitching element such as a TFT (thin film transistor), a liquid crystalcapacitor, and the like. In such a pixel, the TFT has a gate connectedto a scanning signal line and has a drain and source via both of whichone electrode of the liquid crystal capacitor is connected to a datasignal line, and the other electrode of the liquid crystal capacitor isconnected to a common electrode wire that is common to all the pixels.The common electrode driving section 4 supplies a voltage that is to beapplied to this common electrode wire.

In the liquid crystal display apparatus 10, the gate driving section 2selects a scanning signal line, and an image data signal DAT to be sentto a pixel corresponding to a combination of the scanning signal linebeing selected and each of the data signal lines is outputted to thatdata signal line by the source driving section 3. With this, the imagedata are respectively written in pixels connected to the scanning signalline. Similarly, the gate driving section 2 sequentially selects thescanning signal lines, and the source driving section 3 outputs imagedata to the data signal lines. As a result, the image data arerespectively written in all the pixels of the display section 1, so thatthe display section 1 displays an image corresponding to the image datasignal DAT.

On this occasion, the image data that are sent from the control section6 to the source driving section are transmitted as an image data signalDAT in a time-dividing manner. When the image data are sent to thesource driving section 3 via the control section 6, the current-framedata is stored in the frame memory 7. The one-frame data stored in theframe memory 7 is used to be compared with the previous-frame data whenthe calculating section 5 performs overdrive driving.

The source driving section 3 extracts various image data from the imagedata signal DAT at timings based on a source clock signal, an inversionsource clock signal, and a source start pulse each serving as a timingsignal, and then outputs the image data to the respective pixels.

Incidentally, for example, it is known that the response speed of anormally black system becomes low in a transition from a low gradationto a higher gradation. This causes a problem when a moving image isdisplayed. The response speed becomes low especially when both of thegradations (i.e., the gradation before change and the gradation afterchange) are at low levels. On the other hand, it is known that theresponse speed of a normally white system becomes low in a transitionfrom a high gradation to a lower gradation or especially when both ofthe gradations are at high levels.

In view of this, the present embodiment achieves an improvement inresponse speed by a first method of displaying a still image inaccordance with a conventional normal gradation-luminance curve shown inFIG. 13 and displaying a moving image without using a level at which theresponse speed becomes low.

Specifically, assume, for example, that when the total number ofgradations is 256, a normally black system is slow in responding toapplied voltages 0V to 31V respectively corresponding to gradations 0 to31. In this case, the applied voltages V0 to V31 respectivelycorresponding to the 32 gradations are raised to the same voltage as anapplied voltage V32 corresponding to a gradation 32.

This results in such a gradation-luminance relationship as shown inFIG. 1. In addition, the performance of overdrive driving makes itpossible to, as shown in FIG. 3, achieve a very satisfactory improvementin response speed at which a moving image is displayed. Further, sincethe other gradation applied voltages (V32 to V255) are not changed, theγ characteristic of the display section 1 is not changed, so that it ispossible to maintain a good display.

The following explains overdrive driving. As shown in FIG. 4( a), theoverdrive driving is a driving method of applying correction dataderived from a relationship formed by making a comparison between datacorresponding to the current frame and data corresponding to the oneframe that came immediately before the current frame. To be moreaccurate, the relationship refers to “to apply such a gradation as tomake a difference bigger than the difference between the gradation ofthe one frame that came immediately before the current frame (such aframe being hereinafter referred to as ‘previous frame’) and thegradation of data inputted to the current frame”. For example, theoverdrive driving is such driving that in cases where the gradation ofthe previous frame is V0 and the gradation of data inputted to thecurrent frame is V128, the gradation V160 is applied. The application ofsuch a gradation value makes it possible to obtain a liquid-crystalresponse waveform, shown in FIG. 5( b), which has a sharp rising edge.

As described above, the overdrive driving is a driving method ofapplying an unusual voltage only to a frame that comes immediately aftera change in gradation. Further, the amount of change in voltage ischanged in accordance with the relationship between a gradation beforechange and a gradation after change. Therefore, the luminance at agradation is not steadily changed to a certain value.

A gradation value for applying a voltage higher than a normally desiredgradation applied voltage for the purpose of the overdrive driving,i.e., a gradation value that is found in accordance with therelationship between a gradation before change and a gradation afterchange can be obtained by calculation. However, the present invention isnot necessarily limited to this. As shown in FIG. 5, such a gradationvalue can be calculated with use of the look-up table 8.

Incidentally, the luminance-gradation characteristic of FIG. 1 causes arange of luminances that can be displayed to be narrower than whennormal display driving is performed, thereby causing deterioration indisplay quality. That is, gradations other than those equalized arenormal. Therefore, a good γ characteristic is exhibited. However, thenumber of gradations is reduced to the extent of the equalization.

In view of this, the present embodiment smoothens theluminance-gradation characteristic in the following manner.

For example, on the assumption that the total number of gradations is256 and the predetermined gradation is m, the present embodiment sorts ngradations into a range of voltages for gradations (n−m) by a secondmethod.

Specifically, without using applied voltages respectively correspondingto gradations of less than the predetermined gradation m (m being aninteger of not less than 1), (n−m) types of gradation are partiallyoverlapped so that n gradations are obtained, and then the n gradationsare sorted into a range of (i) an applied voltage corresponding to thepredetermined gradation m to (ii) an applied voltage corresponding to agradation (n−1). Then, such overdrive driving is performed that when anapplied voltage for a gradation k (k being an integer of 0 to (n−1))obtained by the sorting is applied, a voltage higher than a voltage thatis normally applied for the k gradation is applied.

This results in a luminance-gradation curve L1 shown in FIG. 6. That is,this luminance-gradation curve L1 covers a gradation range of 1 to 255;therefore, the display quality becomes better than before. However,since the n gradations are expressed in a pseudo manner with use of theremaining (n−m) gradations, the number of gradations is reduced.Further, the γ characteristic is such that a white floating phenomenonoccurs. However, the second method is easily carried out because it canbe realized by using a conventional liquid crystal driver withoutmodification.

Meanwhile, for example, the present embodiment can redivide the samerange of applied voltages in accordance with n gradations by a thirdmethod. Specifically, without using gradations of less than thepredetermined gradation m (m being an integer of not less than 1), atotal of n (n being an integer of more than m) types of gradation areredivided within a gradation range of m to n−1. Then, such overdrivedriving is performed that when an applied voltage for a gradation k (kbeing an integer of 0 to (n−1)) obtained by the redivision is applied, avoltage higher than a voltage that is normally applied for the kgradation is applied.

Although the third method is more complicated than the second method,the third method yields a smoother gradation display. That is, since thegradations are reset, all the n gradations can be expressed. However,the γ characteristic is such that a white floating phenomenon occurs.Further, when the third method is carried out, a conventional liquidcrystal driver cannot be used without modification. This is because theliquid crystal driver needs to be arranged such that gradation voltagescan be changed.

Further, the elimination of these low gradations and the performance ofoverdrive driving make it possible to obtain a response waveform, suchas the one shown above in FIG. 3, which has no angular response(two-step response) portion and has a sharp rising edge.

FIG. 7 specifically shows gradations and liquid crystal applied voltageswith respect to each of the first to third methods. As shown in FIG. 7,all the methods have the same liquid crystal applied voltage when theoriginal data corresponds to a gradation 0; however, they differ fromone another in subsequent processes.

Incidentally, in cases where the gradations are adjusted by theaforementioned processes, the γ characteristic is changed. As a result,an entirely whitish image is obtained in case of a normally blacksystem, and an entirely dark image is obtained in case of a normallywhite system.

In such a case, it is preferable, for example, to perform light controlwith use of a backlight by a fourth method (such light control beinghereinafter referred to as “backlight light control”). This backlightlight control is performed by the backlight driving section 9 of FIG. 2.The following explains the backlight light control with reference to acase where the backlight light control is performed in a normally blacksystem.

That is, the process of rearranging gradations causes such a change ingradation-luminance characteristic that results in a luminance-gradationcurve L1 indicated by a solid line in FIG. 8. FIG. 8 indicates a normalluminance-gradation curve L0 by a dotted line.

Therefore, a reduction in backlight luminance makes it possible toprevent an image from being entirely whitish. In this case, thebacklight luminance can be adjusted so that, as shown by aluminance-gradation curve L2 indicated by a dashed line in FIG. 9, theaverage values of the luminances of all the gradations become equal.Further, the present invention is not limited to this. For example, itis possible to make an adjustment such that the luminances of specificgradations become equal.

Further, although the foregoing explanation assumes that a normallyblack system is employed, the present invention is not necessarilylimited to this. A normally white system can also be employed in thesame line of thought.

That is, it is known that the response speed of a normally white systembecomes low in a transition from a high gradation to a lower gradationor especially when both of the gradations are at high levels. Thiscauses a problem when a moving image is displayed.

Therefore, the response speed can be improved by carrying out a displaywithout using a level at which the response speed becomes low.

Specifically, for example, in cases where the display section 1 having atotal of 256 gradations is slow in responding to gradations V255 toV241, applied voltages respectively corresponding to the 15 gradationsare raised to the same voltage as the gradation V240. This results in agreat improvement in response characteristic.

Further, in cases where the other gradations (V0 to V240) are notchanged, the γ characteristic of the display section 1 is not changed,so that it is possible to maintain a good display.

As described above, the method of the present embodiment for driving theliquid crystal display apparatus 10 is characterized as follows. Thatis, for example, in case of a normally black system, a low voltage canbe applied as a gradation output when a still image is displayed.However, when a moving image is displayed, only a gradation that ishigher by a predetermined voltage is used instead of that low voltage.

That is, a liquid crystal driving circuit generates an applied voltagecorresponding to each gradation, but each gradation voltage is basicallyfixed. According to Japanese Unexamined Patent Application PublicationNo. 78129/2004 (Tokukai 2004-78129), a gradation voltage is set inadvance to be higher by a predetermined voltage. On the other hand,according to the present embodiment, a gradation voltage is set to be atthe same level as a normal voltage, and a gradation of not more than apredetermined voltage is not used when a high-speed response isrequired. This makes it possible to easily realize a high-speedresponse. Further, when a high-speed response is not required, agradation of not more than a predetermined voltage can be used. Thismakes it possible to carry out a display with higher contrast (withhigher luminance in some cases).

Further, the application of the technique of the present embodiment to aconventional driving circuit as well as a liquid crystal displayapparatus, having such a driving circuit, which carries out a display byusing a portion of not less than a predetermined voltage makes itpossible to realize a high-speed response without causing a change inthe driving circuit.

Further, for gradations other than the gradations whose driving voltageshave been uniformed, normal driving is performed. This results in adisplay with a good gradation γ characteristic.

Furthermore, a moving image and a still image are discriminated fromeach other in accordance with some sort of signal that represents amoving image or a still image, and in case of a still image, normaldriving is performed for all the gradations. This makes it possible tocarry out a display without impairing γ characteristic, luminance, andcontrast at all.

Further, an increase in power can be prevented by, at the time ofdisplaying a still image, stopping driving a memory for overdrive,driving a calculating circuit, and supplying power to the memory.

[Embodiment 2]

Another embodiment of the present embodiment will be described belowwith reference to FIGS. 10 and 11. Arrangements other than thosedescribed in the present embodiment are the same as in Embodiment 1.Further, for convenience of explanation, members having the samefunctions as those shown in the drawings of Embodiment 1 are given thesame reference numerals, and will not be described below.

Embodiment 1 rearranges the gradation range; however, the presentinvention is not particularly limited to this. As shown in FIG. 10, theapplied voltages can be simply shifted. This makes it possible to obtaina wide-range luminance characteristic.

Incidentally, this method of simply shifting the applied voltages causesan increase in luminance of all the gradations. Therefore, as withEmbodiment 1, the γ characteristic is changed. As a result, an entirelywhitish image is obtained in case of a normally black system, and anentirely dark image is obtained in case of a normally white system.

In such a case, it is preferable to perform backlight light control aswith Embodiment 1. This backlight light control is performed by thebacklight driving section 9 of FIG. 2. The following explains thebacklight light control with reference to a case where the backlightlight control is performed in a normally black system.

That is, the process of simply shifting the applied voltages causes sucha change in gradation-luminance characteristic that results in aluminance-gradation curve L1 indicated by a solid line in FIG. 10. FIG.10 indicates a normal luminance-gradation curve L0 by a dotted line.Note that FIG. 10 shows a curve that has been simply shifted. However,strictly speaking, because the vertical axis represents the convertedluminance, the curve L1 is not obtained by simply shifting the curve L0.

As described above, a reduction in backlight luminance makes it possibleto prevent an image from being entirely whitish. Specifically, anadjustment of backlight luminance makes it possible that, as shown inFIG. 11, the gradation-luminance characteristic exhibited when a movingimage is displayed is equal to the gradation-luminance characteristicexhibited when a still image is displayed.

Further, although the foregoing explanation assumes that a normallyblack system is employed, the present invention is not necessarilylimited to this. A normally white system can also be employed in thesame line of thought.

As described above, according to the method of the present invention fordriving the liquid crystal display apparatus, it is preferable thatapplied voltages respectively corresponding to predetermined gradationsm to (n−1) for use in displaying a moving image be identical tostill-image applied voltages respectively corresponding to predeterminedgradations m to (n−1) for use in displaying a still image.

With this, as applied voltages respectively corresponding topredetermined gradations m to (n−1), still-image applied voltagesrespectively corresponding to predetermined gradations m to (n−1) foruse in displaying a still image are used. This makes it possible to usea gradation-luminance characteristic that is exhibited when a stillimage is displayed, so that there is no change in display quality.

Further, according to method of the present invention for driving theliquid crystal display apparatus, it is preferable that when the liquidcrystal display apparatus employs a normally black system, the appliedvoltages respectively corresponding to the gradations of less than thepredetermined gradation m should not be used.

This makes it possible to prevent an angular response in overdrivedriving.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus, it is preferable that when all thegradations consist of gradations 0 (black) to 255 (white) and the liquidcrystal display apparatus employs a normally black system, thepredetermined gradation m be defined as 1≦m≦32.

This brings about an effect of improving the response speed of anormally black system when the predetermined gradation m is defined as1≦m≦32.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus 10, it is preferable that when allthe gradations consist of gradations 0 (black) to 255 (white) and theliquid crystal display apparatus employs a normally black system, thepredetermined gradation m be defined as 9≦m≦15.

This brings about an effect of improving the response speed of anormally black system when the predetermined gradation m is defined as9≦m≦15, and also achieves a reduction in deterioration of contrast and areduction in influence of deterioration in image quality. For example,in case of a display whose gradation has a γ characteristic adjusted tobe 2.2 and whose initial contrast is not less than 200, a reduction incontrast at 9≦m≦15 is kept to not more than 30%.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus, it is preferable to adjustbacklight luminance in order to prevent a screen from being entirelywhitish.

Such an adjustment of backlight luminance makes it possible to prevent ascreen from being entirely whitish when the applied voltages areuniformly shifted.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus, it is preferable that appliedvoltages respectively corresponding to predetermined gradations 0 to q−1for use in displaying a moving image be identical to applied voltagesrespectively corresponding to predetermined gradations 0 to q−1 for usein displaying a still image.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus, it is preferable that when theliquid crystal display apparatus employs a normally white system, theapplied voltages respectively corresponding to the gradations of notless than the predetermined gradation q should not be used.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus, it is preferable that when all thegradations consist of gradations 0 (black) to 255 (white) and the liquidcrystal display apparatus employs a normally white system, thepredetermined gradation q be defined as 224≦q≦255.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus, it is preferable that when all thegradations consist of gradations 0 (black) to 255 (white) and the liquidcrystal display apparatus employs a normally white system, thepredetermined gradation q be defined as 241≦q≦247.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus, it is preferable to adjustbacklight luminance in order to prevent a decrease in luminance of anentire screen.

All this makes it possible to provide a method for driving a liquidcrystal display apparatus 10, regardless of whether the liquid crystaldisplay apparatus 10 employs a normally black system or a normally whitesystem, which method makes it possible to prevent deterioration indisplay quality of both a still image and a moving image and to achievean improvement in response speed at which a moving image is displayed.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus, it is preferable to adjust anapplied voltage in accordance with a γ characteristic so that the γcharacteristic is improved.

This achieves an improvement in γ characteristic. Specifically, such agradation that the γ characteristic is improved can be picked up bycalculation based on the transmittance characteristic of liquid crystalswith respect to an applied voltage.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus 10, it is preferable todiscriminate between a still image and a moving image in accordance witha signal for discriminating between a still image and a moving image.

This makes it possible to provide a method for driving a liquid crystaldisplay apparatus 10, which method makes it easy to discriminate betweena still image and a moving image by acquiring a signal fordiscriminating between a still image and a moving image. When a stillimage is displayed, this method makes it possible to perform normaldriving for all gradations, thereby making it possible to display thestill image without impairing γ characteristic, luminance, and contrast.When a moving image is displayed, this method makes it possible toachieve an improvement in response speed.

Further, according to the method of the present invention for drivingthe liquid crystal display apparatus, it is preferable to suspendoverdrive driving when a still image is displayed.

This makes it unnecessary to increase the response speed when a stillimage is displayed, and makes it possible to achieve a reduction inpower consumption by suspending overdrive driving.

The embodiments and concrete examples of implementation discussed in theforegoing detailed explanation serve solely to illustrate the technicaldetails of the present invention, which should not be narrowlyinterpreted within the limits of such embodiments and concrete examples,but rather may be applied in many variations within the spirit of thepresent invention, provided such variations do not exceed the scope ofthe patent claims set forth below.

INDUSTRIAL APPLICABILITY

The present invention can be used as a method for driving a liquidcrystal display apparatus such as an active-matrix display.

The invention claimed is:
 1. A method for driving a liquid crystaldisplay apparatus, comprising the steps of: when a still image isdisplayed, outputting applied voltages to pixels, the applied voltagesrespectively corresponding to a total of n (n being an integer of notless than 4) types of gradation 0 to (n−1); when a moving image isdisplayed, without using applied voltages respectively corresponding togradations of less than a predetermined gradation m (1≦m≦(n−2)),overlapping (n−m) types of gradation partially so that n gradations areobtained and then sorting the n gradations into a range of (i) anapplied voltage corresponding to the predetermined gradation m to (ii)an applied voltage corresponding to the gradation (n−1); and when anapplied voltage corresponding to a gradation k (k being an integer of 0to (n−1)) obtained by the sorting is applied to the pixels, performingoverdrive driving with respect to the total of n types of gradation. 2.The method as set forth in claim 1, wherein when the liquid crystaldisplay apparatus employs a normally black system, the applied voltagesrespectively corresponding to the gradations of less than thepredetermined gradation m are not used.
 3. The method as set forth inclaim 1, wherein n is greater than or equal to 8 and when all thegradations consist of gradations 0 (black) to (n−1) (white) and theliquid crystal display apparatus employs a normally black system, thegradation m is defined as 1≦m≦(n/8).
 4. The method as set forth in claim3, wherein n is
 256. 5. The method as set forth in claim 1, wherein whenn is greater than or equal to 32 and all the gradations consist ofgradation 0 (black) to (n−1) (white) and the liquid crystal displayapparatus employs a normally black system, the gradation m is defined as(n/32)+1m≦{(n/16)−1)}.
 6. The method as set forth in claim 5, wherein nis
 256. 7. The method as set forth in claim 1, comprising the step ofadjusting backlight luminance in order to prevent a screen from beingentirely whitish.
 8. The method as set forth in claim 1, comprising thestep of discriminating between a still image and a moving image inaccordance with a signal for discriminating between a still image and amoving image.
 9. The method as set forth in claim 1, comprising the stepof suspending overdrive driving when a still image is displayed.